Devices for generating sectional images of an object under examination, in particular of a human body or body part, using nuclear magnetic resonance, are known in the art. In this context the body under examination is placed in a strong homogeneous magnetic field, called the background field, which causes in the body an alignment of the nuclear spin of atomic nuclei, especially of hydrogen atom nuclei (protons) bound to water. These nuclei are then excited, by means of high-frequency excitation pulses, into a precessional motion. After a corresponding excitation pulse has ended, the atomic nuclei precess at a frequency which depends on the strength of the background field, and then after a predetermined relaxation time swing back, as a result of their spin, into the preferred direction defined by the background field. From the spatial spin density or from the distribution of relaxation times, an image can be generated with reference to one layer of the body by computer and/or instrumental analysis of the integral high-frequency nuclear signals. The nuclear resonance signal detectable as a result of the precessional movement is allocated to its point of origin by the application of linear field gradients. For this purpose, suitable gradient fields are overlaid on the background field and controlled so that excitation of the nuclei occurs only in one layer that is to be imaged. Image depiction based on these physical effects is also known as nuclear spin tomography or nuclear magnetic resonance tomography.
A transmission (transmitting) device with at least one antenna is required for high-frequency (HF) excitation of the nuclear spin. As indicated by the publication "ntz-Archiv," vol. 11 (1989) no. 5, pages 237 to 243, or by European Application No. 0 073 375, a corresponding antenna can be configured as what is called a "whole-body resonator."
In corresponding devices for nuclear magnetic resonance, it can be advantageous to be able to switch over the field characteristic of a transmission antenna, in terms of location and magnitude, rapidly from one HF pulse to another HF pulse, i.e. during image recording. This requires an HF antenna system that is configured as an array of individual antenna elements which should be largely magnetically decoupled from one another.
A corresponding HF antenna system for reception (receive) is known from International Application No. 89/05115. It consists of a plurality of magnetically decoupled antenna elements, arranged next to one another, that in order to receive magnetic resonance signals are allocated to an examination region. The antenna elements are configured as rectangular conductor loops. For magnetic decoupling thereof, adjacent antenna elements overlap. The signals received by the individual antenna elements are combined into a common signal by taking into account the phase shifts between the individual signals due to the spatial orientation of the antenna elements.
A corresponding HF antenna system for reception purposes is indicated by European Application No. 0 273 484.
Another antenna system is shown in German Application No. 43 31 021, which illustrates an HF antenna system (array) that is made up of waveguide resonators or conductor loops that are decoupled from one another and that are supplied with separate transmission signals that are adjustable in phase and amplitude.